The present invention relates generally to gas separation and, more particularly to the use of integrally skinned asymmetric permeable polyaniline membranes and hollow fibers for separating gases.
Membrane-Based Chemical Separation (MBCS) systems have experienced rapid growth in U.S. industries (e.g., Monsanto, DuPont, Dow, and Union Carbide) over the past two decades and are used to separate and purify many high volume chemicals. Broadly stated, a membrane is a barrier material that allows for selective transport. High-profile chemical separations achieved with state of the art commercial membrane systems include: (1) size dependent particle removal filtration from fluid streams; (2) water desalination by reverse osmosis; (3) organic/organic separations and solvent dehydration by pervaporation; and (4) gas separations for hydrogen recovery, nitrogen production, and acid gas removal from natural gas streams. New materials with higher rates and selectivities are required by industry to advance MBCS technologies. MBCS systems permit: (a) high throughput and high selectivity to efficiently concentrate individual components from complex mixtures; (b) small numbers of mechanical parts; and (c) low capital and operating costs due to significantly reduced energy requirements. Because of these advantages, MBCS systems occupy an important place in separation technologies and are attractive for commercial process operations.
The process of separating pure components from a mixture of gases is of great industrial importance. Current gas separation technologies have several shortcomings which include high capital costs, poor energy efficiency, and generation of secondary pollution. Two principal gas separation techniques in use today are cryogenic distillation and pressure-swing gas adsorption. Membrane-based separations have more recently been proven to be economically competitive with these methods while possessing several advantages including reduced energy costs and large reductions in secondary pollution. Moreover, improvements in membrane separation systems may lead to significant savings for the energy industry. Advanced membrane systems for enriched oxygen and nitrogen production alone could save 0.4 quad units of energy per year. See, e.g., xe2x80x9cMembrane Separation Systemsxe2x80x9d, U.S. Department of Energy, DE 90-011771 (1990). New membrane systems needed to advance gas separation technology should be: (a) environmentally stable; (b) easy to manufacture and maintain; (c) low in cost; and, most importantly, (d) capable of combining high gas selectivity with high gas flux.
Air separation systems based on gas selective polymer membranes have now been commercialized by several U.S. companies. These are important separations since nitrogen and oxygen are the second and third largest commodity chemicals produced in the United States. See, e.g., J. Huggin, Chem. Eng. News 66, 7 (1988). The first commercial air separation systems used conventional engineering polymers for O2/N2 separations with selectivities in the range of 2-4. Current technology employs membranes with separation factors of 6-7, and an oxygen permeability of 2 Barrers. See, e.g., U.S. Pat. No. 4,818,254 for xe2x80x9cSemi-Permeable Membranes Consisting Predominantly Of Polycarbonates Derived From Tetrahalobisphenolsxe2x80x9d which issued to Joginder N. Anand et al. on Apr. 4, 1989. A recent report prepared by the U.S. Department of Energy states that if stable membranes are developed with an oxygen/nitrogen separation factor of 20 and an oxygen permeability of 10 Barrers, such systems would completely displace present day cryogenic air separations. A membrane with these qualities would readily allow compressed air to be enriched to  greater than 75% oxygen or produce nitrogen of  greater than 99.9% purity. Energy costs saved in oxygen and nitrogen production, and those saved from enhanced combustion processes, would equal the energy equivalent to 105 to 106 barrels of oil per day. See, e.g., xe2x80x9cMembrane Separation Systems: A Research Needs Assessmentxe2x80x9d by W. Koros, U.S. Department of Energy, DE 90-011770 (1990). Other important commercial gas separations include the recovery of hydrogen and the separation of nitrogen, helium, hydrogen, and carbon dioxide from natural gas.
The conjugated polymer polyaniline is stable in both its undoped and doped states and has a simple and completely reversible acid/base doping chemistry. Permeability tests on thick (25 to 50 xcexcm) as-cast polyaniline films have shown that gases of different kinetic diameters permeate at different rates leading to high selectivity ratios for important gas pairs such as oxygen/nitrogen, hydrogen/nitrogen, and carbon dioxide/methane. See, e.g., xe2x80x9cConjugated Polymer Films For Gas Separationsxe2x80x9d by M. R. Anderson et al., Science 252, 1412 (1991). By partially doping polyaniline, a highly selective membrane was formed. Two recent papers have confirmed that partially doped polyaniline has the highest known oxygen/nitrogen selectivity and lies above an xe2x80x9cupper boundxe2x80x9d permselective behavior for both glassy and rubbery polymers. See, e.g., S. Kuwabata and C. R. Martin, J. Membrane Sci. 91, 1 (1994) and L. Rebattet et al., J. Appl. Polym. Sci. 57, 1595 (1995). The commercial potential for gas-selective polyaniline membranes has been limited, however, since there are no reports where a sufficiently thin skin ( less than 1 xcexcm) has been generated to achieve commercially significant gas transport rates. Since gas flux is inversely proportional to the thickness of a barrier membrane, the thinnest possible nonporous polymer layer is desired. Suitable membranes must also be able to withstand pressures of up to 100 atmospheres or more. Separations are therefore generally carried out using asymmetric membranes in which a thin layer or skin of polymer is grown on a porous structural support (asymmetric composite), or where both the thin separating layer and the porous support are the same polymer (integrally skinned asymmetric membrane or ISAM).
Accordingly, it is an object of the present invention to provide a robust, stable permeable polyaniline membrane having a significant gas transport rate and significant gas transport selectivity.
Another object of the present invention is to provide a robust, stable permeable polyaniline hollow fiber having a significant gas transport rate and significant gas transport selectivity.
Still another object of the invention is to provide a robust, stable permeable polyaniline membrane having a significant gas transport rate and oxygen/nitrogen, hydrogen/nitrogen and carbon dioxide/methane gas selectivities.
Yet another object of the present invention is to provide a robust, permeable polyaniline hollow fiber having a significant gas transport rate and oxygen/nitrogen, hydrogen/nitrogen and carbon dioxide/methane gas selectivities.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein the method for preparing gas-permeable, integrally skinned, asymmetric membranes (ISAMs) hereof includes: forming a solution having between 15% and 30% w/w of the EB form of polyaniline; pouring the solution onto a substrate forming thereby a membrane; and immersing the membrane in a nonsolvent for polyaniline, thereby causing the EB form of polyaniline to precipitate forming an ISAM having a permeable dense skin in the vicinity of the surface of contact of the membrane with the nonsolvent, and a lower density porous support portion.
It is preferred that the nonsolvent for polyaniline includes water.
Preferably, the permeable dense skin is caulked by applying a coating of polydimethylsiloxane thereto.
In another aspect of the present invention in accordance with its objects and purposes, the method for preparing integrally skinned, gas-permeable hollow fibers hereof includes: forming a solution having between 15% and 30% w/w of the EB form of polyaniline; extruding the solution through the outer bore of a hollow fiber spinnerets nozzle; passing the solution through an air-gap; and subsequently directing the extruded solution into a coagulation bath containing a nonsolvent for polyaniline, whereby a hollow fiber having a permeable dense skin in the vicinity of the surfaces thereof which contact the nonsolvent and a lower-density porous interior support portion is formed.
It is preferred that the nonsolvent for polyaniline includes water.
Preferably, the outer surface of the hollow fiber is caulked by applying a coating of polydimethylsiloxane thereto.
Benefits and advantages of the invention include readily prepared, environmentally stable, high-flux, gas-permeable films and hollow fibers, the gas selectivity of the films and fibers being tunable by doping using various acids and doping levels.